1. Technical Field
This invention relates to multiplexing a plurality of ternary input signals into a common output. More particularly, the invention relates to a ternary signal multiplexor circuit capable of passively multiplexing ternary input signals with minimum feedback and minimum attenuation.
2. Background Art
Multiplexors capable of combining multiple input signals into a common output signal are well known. Multiplexors are used in a variety of equipment, such as power supplies, telephonic equipment, and logic circuits. Some of the problems associated with prior art multiplexors are that: (1) they have poor isolation of input sources, (2) they attenuate the signal, (3) they need an external power source and (4) they are costly due to a requirement for numerous components. Due to the interaction of these problems in prior art multiplexors, solving any one of them has often resulted in exacerbating the others.
The following is a more detailed discussion of the problems identified above. With regard to attempts to improve isolation, they have often resulted in overly complex isolation circuits in each input stage of the multiplexor. There are several disadvantages associated with isolation stages. Not only do they require a significant number of components, but a separate power supply is often required to operate the isolation circuits. These disadvantages result in an increase in both operational and fabrication costs.
Attenuation in prior art multiplexors typically increases as the number of input signal lines are added. For example, a multiplexor with four input lines would produce an output signal with 1/4 the energy of the input signal. Likewise, a multiplexor with eight input lines would produce an output signal with 1/8 the energy of the input signal. As the number of input signal lines increase, the amount of attenuation increases.
Further, the problem of attenuation has often been solved at the cost of adding complex amplifier stages and separate power supplies to the circuit. As a result, the same disadvantages associated with attempts to improve the isolation problem recur in circuits designed to minimize attenuation. Alternatively, minimizing the attenuation by reducing the complexity of the multiplexor often leads to inadequate isolation.
An external power source adds to the cost of the multiplexor, and can limit the physical location of circuits due to a necessity to have the power supply within a reasonable distance of the multiplexor. However, a passive multiplexor can have adverse effects on signal strength.
Utilizing a large number of components to allow for more options in the design of a circuit for isolation purposes results in increasing the circuit size, increasing the power requirements, increasing the attenuation caused by the components in the circuit, and increasing the cost of fabrication.